In many instances, both at surface locations and underground excavations, holes are bored into the underlying surface using in-the-hole ("ITH") or down-the-hole ("DTH") drilling techniques.
ITH drilling is accomplished by employing a pneumatic hammer disposed in the hole bottom at the end of a string of drill pipe. The drill pipe string consists of a plurality of interconnected drill rod sections extending into the hole. The pneumatic hammer and its associated drill bit are attached to the bottom end of the string directly against the bottom of the hole. The hollow drill pipe serves as a conduit for compressed air, supplied at the surface, to be fed to the hammer. The air, typically pressurized to about 270-330 pounds per square inch (1.9-2.3 MPa) and entrained with oil, operates the hammer.
Typically, the standard drill rod is a little over 5 feet (1.5 m) long and 6.5 inches (16.6 cm) in outside diameter. One end of the rod includes a threaded male pin. The opposite end includes a threaded female box. The rods are configured so that a pin end fits into an adjacent box end thereby building up the drill string.
After the hole is drilled a predetermined amount, an additional rod must be added to the string. Every time the string is extended, the drilling process must be interrupted. The cuttings about the hammer must be blown out and up between exterior of the pipe and the bore wall. The compressed air within the string must then be bled out safely so the top connection can be broken in order to add an additional rod.
After the new rod is attached, the string must be recharged with compressed air. A drill string 200-300 feet (61-91 m) long may take up to two minutes or more to be fully charged.
During this lag time, however, the hammer immediately begins to operate by reciprocally driving the hammer piston against the drill bit. Hammers are designed to efficiently operate at the previously mentioned pressures of 270-330 psi (1.9-2.3 MPa). As the air pressure begins to slowly increase the hammer piston commences to strike the bit with insufficient energy to break the rock. This ineffectual flailing action generates debilitating heat. Without the proper air pressure, the cooling/lubricating effect of the air/oil mixture is reduced. The cutter temperature may rise in excess of 180.degree. F. (82.2.degree. C.). Although not yet fully operational, this early inefficient percussion causes the tool to expand. Expansion sometimes causes the carbide cutter buttons to fall off. Unfortunately, there is no equipment currently available to disable the hammer until the pressure comes up to standard.
Each time the string is extended, undesirable drilling inefficiencies are added to the operation. Time delay, increased bit wear, hole inaccuracy and additional maintenance add to the expense of drilling deep holes.